evanescent mode circular waveguide filtersk5tra.net/tech library/filters - general/wg filters...3...
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K5TRA
EVANESCENT MODE CIRCULAR WG FILTERS
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2K5TRA
MOTIVATION: NEED IMAGE FILTERS for X-BAND TEST-SET
Splitter
Image
Filter
Image
Filter
11.250 GHz
DRO
IF
Port -1
IF
Port -2
RF
Port -1
RF
Port -2
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3K5TRA
X-BAND TEST-SET CONFIGURATIONS
S21 MEASUREMENT
S11 MEASUREMENT
Splitter
Image
Filter
Image
Filter
11.250 GHz
DRO
IF
Port -1
IF
Port -2
RF
Port -1
RF
Port -2
DUT
Splitter
Image
Filter
Image
Filter
11.250 GHz
DRO
IF
Port -1
IF
Port -2
RF
Port -1
RF
Port -2
Directional
Coupler
Measurement
Port
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4K5TRA
X-BAND TEST-SET
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5K5TRA
LADDER FILTER BASIC BUILDING BLOCK
• Passband: Z1 short and Z2 open
• Stopband: Z1 open and Z2 short
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6K5TRA
J and K INVERTERS PROVIDE REUSE OF RESONATOR TYPE
• Impedance inverter (K) with a series resonator behaves like a
parallel resonator
• Admittance inverter (J) with a parallel resonator behaves like a
series resonator
• Impedance/admittance inverter interface between similar
resonators provides maximum stopband attenuation
• Most common impedance inverter is transmission line that is an
odd multiples of l/4
Z INVERTER Y INVERTER
K J
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7K5TRA
IMPEDANCE/ADMITTANCE INVERTERS
• Impedance (or admitance) inverters
can be used to convert parallel
resonance to a series resonance
characteristic.
• The canonic impedance inverter is the
l/4 line.
• LC forms provide moderate bandwidth
Z inversion.
• Capacitive T and p sections are for
narrow band applications. Negative C
or L is absorbed into resonator
(cancels some positive C or L).
4,0
l Z
Transmission Line
J (or K) Inverter
CZ
0
0
1
- C
C
- C C
- C - C
LZ 00
L
- L - L
- L
L
- L
LCC
LZ
1, 00
L
C C
L L
C
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8K5TRA
BANDPASS FILTER STRUCTURE WITH J INVERTERS
• Filters are formed as cascade of parallel resonators
and inverters
• How do we realize a structure like this in WG?
J J J J
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9K5TRA
BANDPASS FILTER WITH INDUCTIVE J INVERTERS
• Negative inductors of inverter cancel some of the resonator
inductance
• Inverter admittance/impedance sets coupling between
resonators. In this case, coupling is set by ( L)-1
Y INVERTER
J
L
L" L"
L
- L - L
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10K5TRA
EVANESCENT WG MODEL
• Operation BELOW TE11 cutoff
frequency
• Propagation falls off quickly
• Behavior is reactive (inductive)
Ls
Lp Lpa
l
COPPER PIPE
CIRCULAR WG
𝛾 =2𝜋
𝜆0
𝐹𝑐𝐹0
2
− 1
𝑋0 ~ 599 𝛺
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11K5TRA
EVANESCENT WG SECTIONS PROVIDE J INVERTER and RESONATOR L
• RESONATOR C CAN BE ADDED WITH TUNING SCREWS
• SCREW SPACING SETS Ls (SETS COUPLING)
Ls
Lp Lpa
l
COPPER PIPE
CIRCULAR WG
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12K5TRA
EVANESCENT WG FILTER CONSTRUCTION
• 4-40 tuning screws
• Tapped holes and soldered
brass nuts
• Stainless locking nuts
• Coupling loop with 2-56 tuning
• 0.25” 4-40 stand-off mounting
• Operation BELOW
cutoff frequency
• 0.5” Cu pipe for X-band
(actual ID = 0.565”)
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13K5TRA
EVANESCENT WG FILTER ANALYSIS 6th ORDER EXAMPLE
INDUCTOR Qo FOUND EMPERICALLY
TO BE APPROXIMATELY 200
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14K5TRA
DESIGN TABLES N= 2, 3, & 4
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15K5TRA
DESIGN TABLES N= 5, & 6
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16K5TRA
EVANESCENT WG FILTER INSERTION LOSS, 2 dB/DIV
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17K5TRA
EVANESCENT WG FILTER INSERTION LOSS, 5 dB/DIV
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18K5TRA
EVANESCENT WG FILTER -40 dB BW = 705 MHz
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19K5TRA
EVANESCENT WG FILTER USED IN K5TRA X_BAND TEST SET